1. Field of the Invention
The invention relates generally to the hydraulic control of downhole tools and, particularly to methods and devices for determining the state of such hydraulically-actuated tools.
2. Description of the Related Art
Production of hydrocarbons from a downhole well requires subsurface production equipment to control the flow of hydrocarbon fluid into the production tubing. Typical flow control equipment might include a sliding sleeve valve assembly or other valve assembly wherein a sleeve is moved between open and closed positions in order to selectively admit production fluid into the production tubing. The valve assembly is controlled from the surface using hydraulic control lines or other methods.
In a simple system, a sleeve valve would be moveable between just two positions or states: fully opened and fully closed. More complex systems are provided where a well penetrates multiple hydrocarbon zones, and it is desired to produce from some or all of the zones. In such a case, it is desirable to be able to measure and control the amount of flow from each of the zones. In this instance, it is often desirable to use flow control devices that may be opened in discrete increments, or states, in order to admit varying amounts of flow from a particular zone. Several “intelligent” hydraulic devices are known that retain information about the state of the device. Examples of such devices include those marketed under the brand names HCM-A In-Force™ Variable Choking Valve and the In-Force™ Single Line Switch, both of which are available commercially from Baker Oil Tools of Houston, Tex. These devices incorporate a sliding sleeve that is actuated by a pair of hydraulic lines that move the sleeve within a balanced hydraulic chamber. A “J-slot” ratchet arrangement is used to locate the sleeve at several discrete positions that permit varying degrees of fluid flow through the device.
Because these devices are capable of being controlled between multiple states, or positions, determination and monitoring of the positions of the devices is important. To date, position determination has been accomplished by measurement of the amount of hydraulic fluid that is displaced within the control lines as the device is moved between one position and the next. Measuring displacement of hydraulic fluid will provide an indication of the particular state that the tool has moved to because differing volumes of fluid are displaced during each movement. In some instances, however, such as with a subsea pod, it may not be possible to measure fluid volume. Also, the fluid volume measurement technique may be inaccurate at times for a variety of reasons, including leaks within the hydraulic control lines and connections or at seals that lead to fluid loss, which leads to an incorrect determination of position. In addition, the hydraulic control lines may expand under pressure (storage effects) or become distorted due to high temperatures within the wellbore. In long lines, the additional storage volume in such expansion/distortion may be larger than the normally small differences in fluid volume between different movements and lead to inaccurate determinations of position.
The present invention addresses some of the problems of the prior art noted above.